DC-DC CONVERTER CIRCUIT AND INDUCTIVE-LOAD DRIVING APPARATUS USING THE SAME

Abstract: An inductive load drive device using a DC-DC converter circuit with which a high voltage is obtained by the discharge of electromagnetic energy stored in a coil to a capacitor, in which a bias is applied to the magnetic core of a coil (L) with the use of a permanent magnet (Mg) in the direction opposite to the magnetic flux induced by a current so as to increase the electromagnetic energy stored in the coil (L) and make it possible to perform efficient discharge with a small coil.

Description: TECHNICAL FIELD This invention relates to a DC-DC converter circuit. BACKGROUND ART High-speed actuation of a solenoid valve or another such electromagnetic actuator generally requires that an excitation current be raised quickly so that it overpowers an inductance. If we let an internal resistance of a coil be R and an inductance be L, it is known that the transfer function G (S) of an excitation current I with respect to an applied voltage E is G (S) = (1/R) · (1/(1 + L · S/R)) and it is known that, as is clear from the formula, the slope of the rise of the current I immediately after the voltage E has been applied in a state in which I = 0 is E/L and the steady-state current is E/R, and that primary lag in the time constant L/R occurs. Therefore, the applied voltage E must be increased in order to raise the current to a coil of given R and L quickly and thereby achieve fast actuation. However, an increase in the applied voltage E is accompanied by a necessary increase in the steady-state current as well, which tends to lead to heat being generated in the coil, burning, and so on, and the device is also bulkier and wastes more energy. Also, with a machine that makes use of an on-board battery as its power source, such as a moving vehicle, the applied voltage is limited, and sufficient voltage often cannot be obtained. In order to solve this problem, a voltage elevation circuit (such as a flyback type of DC-DC converter) for raising the applied voltage to the coil and a current control circuit for restricting the steady-state current are provided, a high voltage is applied and the current quickly increased during current rise, and the applied voltage is suppressed and the current prevented from increasing more than necessary by means of the current control circuit when the current reaches a specific value. Figure 17 shows an example of a conventional inductive load drive device in which a flyback type of DC-DC converter is used as the voltage elevation circuit. In the figure, 10 is a charger circuit composed of the flyback type of DC-DC converter. A bulkier apparatus is one of the problems encountered when a flyback type of DC-DC converter is used as a voltage elevation circuit. It was common practice in the past to use a choke coil or a transformer as the inductance of the charger circuit for storing energy, but this resulted in an apparatus that was bulkier and more complicated. In EP 0 243 862 A2 an electrical device is disclosed having means for rectifying an AC-current in a DC-current. When the DC-current is switched on and off by a switch resulting an AC-current with a certain off-set. The electromagnetic coil has a magnetic core which is biased by at least one permanent magnet. Due to the pre-magnetizing of the transformer core by means of the permanent magnet a large part of the hysteresis curve, which is usually not being used, can be used at least in the outer sections. An object of this invention is to solve the above mentioned problems and to provide a DC-DC converter circuit that is relatively compact and lightweight, has a simple circuit structure, and is efficient, and to provide an inductive load drive device in which this DC-DC converter circuit is used. This object is solved by a DC-DC converter as defined in claim 1. SUMMARY OF THE INVENTION This invention is a DC-DC converter circuit provided with a power source, a coil that is connected to the power source, switching means for opening and closing a closed circuit including the power source and the coil, a capacitor and a diode, wherein the capacitor is connected in parallel to the switching means and the diode is provided between the coil and the switching means in such a manner as to prevent a current to flow through the switching means from the capacitor when the switching means is closed, wherein the switching means is closed so that a power source voltage will be applied to the coil, which results in energy being stored in the coil, and the switching means is opened at an arbitrarily determined timing, which results in the energy stored in the coil being stored in the capacitor and output, wherein a magnetic core of the coil is biased by a constant magnetic field in a direction of a magnetic flux induced by a current supplied from a power source so that the magnetic core of the coil being not applied with the current is magnetically saturated by the magnetic bias. By thus applying a bias to the magnetic field of the coil and shifting the actuation point, the energy density per unit of surface area of the magnetic core can be increased, and the energy stored in the coil can be increased. Therefore, charging of the capacitor can be carried out efficiently even if a relatively small coil is used, and an inductive load drive device in which a compact and lightweight DC-DC converter circuit is used can be realized. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram of a first practical example of a DC-DC converter circuit of the present invention, in which a single inductance is used and biased by a permanent magnet; Figs. 2(a) to 2(c) are diagrams of the energy density being increased by the application of a bias to the magnetic core; Fig. 3 is a circuit diagram of a another practical example of the DC-DC converter circuit of the present invention, in which a bias is created with an electromagnet; Fig. 4 is a circuit diagram of a practical example of an inductive load drive device in which the DC-DC converter circuit of the present invention is used; Fig. 5 is a circuit diagram of another practical example of the inductive load drive device of the present invention; Fig. 6 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 7 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 8 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 9 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 10 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 11 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 12 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 13 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 14 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 15 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 16 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; Fig. 17 is a circuit diagram of yet another practical example of the inductive load drive device of the present invention; and Fig. 18 is a circuit diagram of a conventional example of an inductive load drive device. BEST MODE FOR CARRYING OUT THE INVENTION Practical examples of the DC-DC converter circuit pertaining to the present invention, and of inductive load drive devices in which this DC-DC converter circuit is used, will be described with reference to the figures. Fig. 1 shows the first practical example of the DC-DC converter circuit of the present invention. The operation of this circuit will be described with reference to the figure. A switch Sw is turned on and off at a certain timing. In a closed circuit A composed of a power source E , a coil L , and the switch Sw , when the switch Sw is turned on, the current that flows through the coil L causes energy to store in the coil L . When the switch Sw is then turned off, the current path shifts to a closed circuit B composed of the power source, the coil L , and a capacitor C , and the instant that the switch Sw is turned off, a high voltage is generated at both ends of the coil L with a diode D side as the high potential side, and the capacitor C begins to be charged through the diode D . The diode D is inserted for the purpose of preventing the short-circuiting of the capacitor C when the switch Sw is then turned on. As the switch Sw is repeatedly turned on and off, the capacitor C is gradually charged to a high voltage, and more energy is stored in the capacitor C . In this practical example, a bias is applied by means of a permanent magnet Mg to the magnetic core of the coil L that is used to charge this capacitor C in the direction opposite to the direction of the magnetic flux generated by the passage of current, which shifts the actuation point of the B-H curve and allows more energy to be stored. This principle is illustrated in Fig. 2. Fig. 2(a) shows a typical B-H curve of a coil. For the sake of simplicity, this will be described in model form as in Fig. 2(b). When the flow of current at this time has raised the flux density to point a , the energy stored in the coil is the Wa portion indicated by the hatching. If a bias is applied to this coil and the actuation point is shifted, then more energy can be stored, as shown by the hatched portion in Fig. 2(c). The method adopted in this practical example to shift the actuation point is to apply a bias to the magnetic core of the coil L through the use of the permanent magnet Mg . As a result, the energy density per unit of surface area of the magnetic core is raised, and the coil L that is used to charge the capacitor C can be more compact and lightweight while the same amount of energy is still obtained. If a coil of the same size is used, then more energy can be obtained with a single switching. The permanent magnet Mg was used as the method for applying the bias in the first practical example, but it is clear that the same effect will be seen if an electromagnetic Me is used instead. An example of this is shown in Fig. 3. Fig. 3 is an example of using a single inductance. Fig. 4 shows a practical example of the inductive load device of the present invention in which the DC-DC converter shown in the above first practical example is used. In Fig. 4, a comparator Co1 is used to manage the charging voltage of the capacitor C . The comparator Co1 compares the terminal voltage of the capacitor C with a reference voltage Vc, and if the voltage of the capacitor C is lower than the reference voltage Vc, an interrupt signal C1 is input to the switch Sw1 to open and close the switch Sw1, and the energy of the coil L is charged into the capacitor C . At the start of drive, when current flows to the inductive load Ll , the switch Sw2 is closed and the voltage that has been charged into the capacitor C is used as the drive commencement voltage. A structure such as this allows an improvement in the rise of the current that flows to the inductive load Ll . A comparator can also be used to manage the value I of the current that flows to the inductive load Ll . For example, a resistor R is inserted serially in the inductive load Ll , current detection means Cd for detecting the voltage at both ends of the resistor R , for example, is provided, and a comparator Co2 compares the detected value with a reference value Vs and determines the current value I. If the current value I is greater than the reference value, the switch Sw2 is opened, and if it is less than the reference value, the switch Sw2 is closed. Repetition of this operation allows the current that flows to the inductive load Ll to be controlled. A circuit that controls the opening and closing of a switch based on an evaluation of the conditions is hereinafter called a switch control circuit Chp . Fig. 5 shows another practical example of the inductive load device of the present invention. In Fig. 5, this circuit comprises a first closed circuit A capable of supplying current from a battery or another such power source E directly to an inductive load Ll , a capacitor C whose negative side terminal is grounded, a charger circuit 10 with which this capacitor C can be charged from a battery or another such power source E with a specific voltage Vc that is higher than the power source voltage E , a second closed circuit C that includes the capacitor C and the charger circuit 10 , a third closed circuit B capable of supplying current from the capacitor C to the inductive load L , and a switch Sw2 with which the first closed circuit A and the third closed circuit B can be opened and closed. The switch Sw2 is turned on and off in the following sequence at a timing controlled by a control means. First, the switch Sw2 is turned off when no current flows to the inductive load Ll . With the closed circuit C , the action of the charger circuit 10 results in the capacitor C being charged with the specific voltage Vc. In the figure, the charger circuit 10 is depicted as the one in Practical Example 1, but the charger circuit is not limited to this. Next, the switch Sw2 is closed at the point when the excitation current I begins to flow, when it is desirable for the excitation current I to the inductive load Ll to rise quickly. As a result, the voltage Vc at both ends of the capacitor C is applied to the inductive load Ll , and the charge stored in the capacitor C causes the excitation current I to go up rapidly. When the switch Sw2 is closed and the charge stored in the capacitor C flows to the inductive load Ll , there is a reduction in the voltage of the capacitor C . When the voltage Vc of the capacitor C becomes equal to the power source voltage E , the diode Da is turned on, the closed circuit A works, the power source voltage E is directly applied to the inductive load Ll through the diode Da , and the current flowing to the inductive load Ll is maintained. Therefore, there is no further reduction in the voltage of the capacitor C . Repetition of this series of operations allows the inductive load Ll to be driven at high speed, and allows the time lag to be eliminated from the current rise time. The control of the current I flowing to the inductive load Ll can also be accomplished by opening and closing the switch Sw2 with a combination of the same detection means Cd and switch control circuit Chp as in the example in Fig. 4. Fig. 6 shows yet another practical example of the inductive load drive device of the present invention. The inductive load drive device in this practical example has a switch Sw3 provided in addition to the switch Sw2 in the practical example in Fig. 10. The switch Sw3 is placed between the inductive load Ll and the GND, and a diode Df is provided between the switch Sw3 and the capacitor C that is used for charging from the connection point of the inductive load Ll and the switch Sw3 , thus creating a circulating path. With this circuit, the switch Sw2 and the switch Sw3 are turned on simultaneously during current rise. In order to control the current I flowing to the load Ll to the set value, the switch Sw3 is turned off when the current I is greater than the set value, and is turned on when the current I is smaller, so that the current value conforms to the set value. After the period in which the load Ll is driven is finished, the switch Sw2 and the switch Sw3 are turned off simultaneously. At this time the energy of the current I flowing through the load Ll can be circulated through the diode Df to charge the capacitor C , which allows the power consumption to be reduced and the overall efficiency increased. Z in the figure is a voltage control means that is used when excess voltage is applied to the switch Sw3 . Fig. 7 shows yet another practical example, in which a regenerative pumping capacitor is provided to a circuit in which the energy of the current I flowing through the load Ll is circulated. The operation here will be described in reference to Fig. 7. The capacitor C1 is charged at a high negative voltage with respect to the GND by a charger circuit. The switch Sw3 is the switch that makes up the load circulation path, and is always closed during load drive. Closing the switch Sw2 during the rise of the current with respect to the load Ll allows the high voltage of the capacitor C1 to be applied to the load Ll . When normal drive is performed, only the switch Sw1 is closed, the power source voltage E is applied directly to the load Ll , and the current is increased. If at this time the switch Sw3 is opened and closed in synchronization with the switch Sw1 and the switch Sw2 , there is no need for the diode D2 , but in actual practice there is a certain amount of discrepancy in the synchronization, and if any kind of overlap is produced, there will be a short circuit between the GND and the power source from the switch Sw3 through the switch Sw1 or the switch Sw2 , so the diode D2 is provided for the safety. Let us now consider a case in which the current I of the load Ll is reduced. When the switch Sw3 is closed and the switches Sw1 and Sw2 are opened, the current circulates through the switch Sw3 , and the energy stored in the inductive load Ll is consumed by being converted into heat by the resistance inside the circulating path, etc. Here, if the switch Sw3 is also opened, the energy stored in the inductive load Ll will charge the regenerative pumping capacitor C2 . Proper selection of the capacitance of the capacitor C2 allows the rate of increase of the terminal voltage of the capacitor C2 per unit of time to be selected. This is combined with the required rate of decrease in the load current I to select a capacitance, which allows the capacitor C2 to be quickly charged to over the power source voltage, and the energy stored in the inductive load Ll is also absorbed. When the terminal voltage of the capacitor C2 is over the power source voltage, reclosing the switch Sw3 allows the energy stored in the capacitor C2 to be readily recovered in the power source E . Therefore, the opening and closing of the switch Sw3 results in the energy stored in the inductive load Ll being regenerated at the power source E . Fig. 8 shows yet another practical example of the inductive load drive device of the present invention, which is a circuit in which switches Sw5 and Sw6 and diodes D2 and D3 have been added to the circuit in Fig. 7. The operation during current rise with respect to the inductive load Ll is the same as in the practical example shown in Fig. 7. When the rate of reduction in the load current I is to be increased, the energy stored in the inductive load Ll will be quickly charged into the regenerative pumping capacitor C2 when the switch Sw5 is opened, and the terminal voltage of the regenerative pumping capacitor C2 will quickly increase. Once the terminal voltage has exceeded the power source voltage, the energy stored in the regenerative pumping capacitor C2 will be regenerated at the power source when the switch Sw5 is closed, and the voltage of the regenerative pumping capacitor C2 will fall to the power source voltage. Thereafter, the load current I is also continuously regenerated at the power source through the diode D3 . As a result, control of the opening and closing of the switch Sw5 allows the energy stored in the inductive load Ll to be regenerated at the power source at a desired rate. Fig. 9 shows yet another practical example of the inductive load drive device of the present invention. In this practical example, a circuit that monitors the terminal voltage of the regenerative pumping capacitor C2 , compares this with a reference value, and controls the timing of the opening and closing of the switch Sw5 according to the comparison results is added to the circuit in Fig. 8. When the terminal voltage of the regenerative pumping capacitor C2 is lower than the reference value, the switch control circuit Chp opens the switch Sw5 and charges the capacitor C2 with the load current I. When the terminal voltage of the capacitor C2 is higher than the reference voltage, the switch control circuit Chp closes the switch Sw5 and regenerates the energy stored in the capacitor C2 at the power source E . In this example, the opening and closing of the switch Sw5 is controlled automatically based on the terminal voltage of the capacitor C2 . Figs. 10 and 11 show still other practical examples of the inductive load drive device of the present invention. Instead of the energy stored in the regenerative pumping capacitor C2 being regenerated at the power source E , it is discharged by a discharging resistor R in Fig. 10, and is discharged by a voltage limiting element (such as a Zener diode or a ZNR) in Fig. 11. Fig. 12 shows yet another practical example of the inductive load drive device of the present invention. In this practical example, instead of the energy stored in the regenerative pumping capacitor C2 being regenerated at the power source E , it is regenerated at a charging capacitor C1 of the charger circuit. When all of the switches Sw1 through Sw5 are opened, the regenerative pumping capacitor C2 is charged by the current flowing through the inductive load Ll . When the charging voltage of this capacitor C2 is higher than the charging voltage of the charging capacitor C1 of the charger circuit, opening the switch Sw5 allows the energy stored in the capacitor C2 to be regenerated at the capacitor C1 . Thus regenerating the energy at the capacitor C1 rather than at the power source E allows the energy to be used without being wasted during the subsequent drive of the inductive load Ll . Fig. 13 shows yet another practical example of the inductive load drive device of the present invention. In this practical example, the power source E is applied by the switch Sw1 to the inductive load Ll via a resistor R1 that serves as a first current control means, and the charging capacitor C of the charger circuit is connected directly to the inductive load Ll without going through a resistor or the like. A resistor R2 is a second current control means that is positioned in the circulating path of the inductive load Ll , and serves to absorb the energy stored in the inductive load Ll when the switches Sw1 and Sw2 are both opened. Fig. 14 shows yet another practical example of the inductive load drive device of the present invention. As can be seen from Figs. 13 and 14, with the series circuit in this practical example that works through the action of the switches Sw1 and Sw2 , the overall circuit will function no matter where the various constituent elements are located on the circuit. In Fig. 13, the structure is such that the positive side of the charging capacitor C is grounded and negative voltage is charged. Fig. 15 shows yet another practical example of the inductive load drive device of the present invention. In this practical example, the circulating path of the load current goes through the power source E when the switches Sw1 and Sw2 are both open, which means that an independent circulating path is omitted and the circuit is simplified. Here, a voltage limiting element Zd is used as the current limiting element, and this voltage limiting element Zd is installed in parallel with the switch Sw1 . Setting the limiting voltage value of this voltage limiting element Zd higher than the power source voltage allows the load current to be reduced all the way down to zero upon completion of the inductive load Ll circulation. The voltage limiting element Zd may also be installed in parallel with the switch Sw2 , but since this blocks the current from the charging capacitor C , the limiting voltage value has to be raised to a higher level, so efficiency suffers. Fig. 16 is a practical example in which a junction type of transistor is employed as the switching means. A logic circuit is employed as the circuit that controls the switching means. The circuit is actuated when a positive input signal is sent to the input IN , a high voltage application switch Tr2 is turned on for a specific time determined by the resistor R2 and the capacitor C2 , and a power source E application switch Tr1 is turned on at the same time. The diode D1 at this time prevents the high voltage on the load side from flowing back to the power source. If the high voltage application switch Tr2 is opened after the specific time has elapsed, the load current will be reduced by the resistor R and will become a steady-state drive current. Once the input IN reaches 0 V, the switches Tr1 and Tr2 are opened unconditionally, the load current circulates through the circulating path including the voltage limiting element Zd , and the energy is quickly absorbed and attenuated by the voltage limiting element Zd . Fig. 17 is yet another practical example of the inductive load drive device of the present invention. The negative side of the charging capacitor C is connected to the positive side of the power source E , and the capacitor voltage is added to the power source voltage, so a higher voltage is obtained and efficiency improves. As described above, with the present invention a bias is applied to the magnetic core of a coil through the use of a permanent magnet or an electromagnet, and a magnetized coil whose energy density per unit of surface area of the magnetic core has been increased is used to charge a capacitor with a DC-DC converter that makes up the charger circuit of an inductive load drive device, so the coil can be made more compact and lightweight in order to obtain the same energy. Also, if a coil of the same size is used, more energy can be obtained with a single switching. Therefore, the charger circuit can be made more compact, lightweight, and efficient, and the inductive load drive device itself can in turn be made more compact, lightweight, and efficient. INDUSTRIAL APPLICABILITY With the present invention, a bias is applied to the magnetic core of a coil through the use of a permanent magnet or an electromagnet, the energy density per unit of surface area of the magnetic core is increased, and the DC-DC converter that makes up the charger circuit of an inductive load drive device is made more compact and lightweight. Such an inductive load drive device is effective for the drive of large on-board motors in cranes and the like, as well as for the drive of all inductive loads.

Claims: A DC-DC converter circuit provided with a power source (E), a coil (L) that is connected to the power source (E), switching means (SW) for opening and closing a closed circuit (A) including the power source (E) and the coil (L), a capacitor (C) and a diode (D), wherein the capacitor (C) is connected in parallel to the switching means (SW) and the diode (D) is provided between the coil (L) and the switching means (SW) in such a manner as to prevent a current to flow through the switching means (SW) from the capacitor (C) when the switching means (SW) is closed, wherein the switching means (SW) is closed so that a power source voltage will be applied to the coil (L), which results in energy being stored in the coil (L), and the switching means (SW) is opened at an arbitrarily determined timing, which results in the energy stored in the coil (L) being stored in the capacitor (C) and being output, characterized in that a magnetic core of the coil (L) is biased by a constant magnetic field (Mg; Me) in a direction of a magnetic flux induced by a current supplied from a power source (E) so that the magnetic core of the coil being not applied with the current is magnetically saturated by the magnetic bias. A DC-DC converter circuit as defined in Claim 1, wherein the magnetic core includes a permanent magnet (Mg), and the magnetic core is magnetically biased by the permanent magnet (Mg) in the direction opposite to the direction of the magnetic flux induced by the current supplied from the power source. A DC-DC converter circuit as defined in Claim 1, wherein the magnetic core has a bias used winding (Me), and the magnetic core is magnetically biased in the direction opposite to the direction of the magnetic flux induced by the current supplied from the power source by the supply of a constant current from a constant current source to the bias used winding.

Inventors: HAGIWARA MASAO, MAEDA KENICHIRO, MORITOKI MASAKAZU, NAKAYAMA TETSUYA, YOSHIDA DAISUKE, HAGIWARA, MASAO, MAEDA, KENICHIRO, MORITOKI, MASAKAZU, NAKAYAMA, TETSUYA, YOSHIDA, DAISUKE, HAGIWARA, Masao, c/o Electr. Div. of Komatsu Ltd., MAEDA, Kenichiro, c/o Electr. Div. of Komatsu Ltd., NAKAYAMA, Tetsuya, c/o Electr. Div. of Komatsu Ltd, YOSHIDA, Daisuke, c/o Electr. Div. of Komatsu Ltd.
Applicants: KOMATSU MFG CO LTD, KOMATSU LTD.
Application year: 1994
Main classification: H02M3
IPC codes: H01F7, H02M3